FIELD OF THE INVENTION
The present invention relates to a bright annealing process in a box annealing furnace for preventing temper colors which are produced on the outermost layer of a cold rolled steel sheet during its annealing.
In the bright annealing, the annealing is done without oxidation of the cold rolled steel sheet so as to maintain its surface brightness, and for this purpose, it is necessary to enclose the cold rolled steel sheet in a furnace atmosphere of such as DX, NX and HNX all through the process including the heating step and the cooling step.
However, the furnace atmosphere of such as DX and NX etc. varies in its equilibrium composition depending on the gas concentration and produces temper colors on a cold steel sheet which is heated and cooled in the atmosphere, thus damaging a considerable portion of the cold rolled steel sheet surface. Not only the temper colors damage the surface appearance of the sheet, but the surface on which the temper colors are produced gives only a rougher surface even after acid-pickling as compared with the surface which is not affected by the temper color and when it is subjected to a phosphate treatment it gives surface irregularlities due to its difference in reactivity from that of the surface which is not affected by the temper color.
Particularly, a high strength cold rolled steel sheet (Si-Mn-Cr steel), a low-grade silicon steel (ZNC-3), and a Riband steel which have been developed with the advent of safety automobiles in recent years have far much larger susceptibility to the temper colors as compared with an ordinary low carbon steel (SPC steel).
Thus, a steel sheet containing a large amount of Si and Mn has large susceptibility to the temper colors during the annealing because Si and Mn themselves are far more easily oxidized than Fe, and temper colors which are produced at a relatively high temperature between 650.degree. and 750.degree. C. are oxides of Mn and Si such as MnSi0.sub.3, which are oxidized under the presence of a small amount of water vapor and become milky white. Further, in a cooling furnace with the same dew point of -60.degree. C., the steel sheet containing a large amount of Mn and Si has a higher temperature range in which the temper color takes place as compared with the ordinary carbon steel sheet, and the temper colors thus produced are brown.
The present inventors have conducted extensive studies to find technical means for minimizing the thickness of the oxide film which causes the temper colors on an assumption that it would be possible to reduce the oxide film thickness to a thickness of no visual problem, although it would be impossible to prevent completely the oxidation of Mn and Si, and have found that the visual oxides, namely the temper colors, which are produced when the cold rolled steel sheet of an ordinary composition or containing Si, Mn and Cr, etc. is annealed under the presence of dew or H.sub.2 O can be prevented by placing an oxidizable material in the annealing process, particularly in a lower temperature zone in the cooling step.
The feature of the present invention lies in that an oxidizable material is placed at a position of a low temperature zone in a circulating path of a furnace gas in a box annealing furnace.